# Is the disjoint union of two Matroids a Matroid itself?

Let $E_1$ and $E_2$ be two disjoint sets.

Moreover, assume that $(E_1, S_1)$ and $(E_2, S_2)$ are matroids.

Define $S:=\{X\cup Y | X\subseteq S_1 \text{ and} Y \subseteq S_2\}$.

S1,S2 and S are independent sets of Matroid.

Prove that $(E_1 \cup E_2, S)$ is a matroid.

• Welcome to stackexchange. If you edit your question to show what you have tried and where you are stuck you are more likely to get answers rather then downvotes or votes to close. Nov 4, 2017 at 23:53
• @EthanBokler yes you are right. I actually changed a typo Nov 4, 2017 at 23:55
• I'm not sure what you're thanking me for, since you did not in fact edit your question to show what you tried. In the meanwhile, you did get an answer. If it's a good one for you be sure to accept it (the check mark). Nov 5, 2017 at 0:09
• What are the $S_i$ supposed to be? This matters because there are many different ways to define a matroid (from their bases, independent sets, flats, circuits, cocircuits, hyperplanes, etc. etc.) and the validity of the result you want to prove depends on which definition(s) we are working with. It seems from the accepted answer that you assume the $S_i$ are the sets of independent sets. For clarity, you should add that assumption to your question. Nov 5, 2017 at 12:46
• @AaronDall yes, they are sets of independent sets. I already edited the question now. Nov 5, 2017 at 13:30

1. Clearly $\emptyset \in S$.
2. Suppose $A' \subseteq A \in S$. Then $A = X \cup Y$ for $X \in S_1$, $Y \in S_2$, so $$A' = A' \cap A = (A' \cap X) \cup (A' \cap Y).$$ But $A' \cap X \subseteq X$, so $A' \cap X \in S_1$; similarly $A' \cap Y \in S_2$. Thus $A' \in S$.
3. Let $A, B \in S$, $|A| > |B|$. Write $A = X \cup Y$, $B = X' \cup Y'$. Then we must have $|X| > |X'|$ or $|Y| > |Y'|$; suppose we have $|X| > |X'|$. Then there is $x \in X$ such that $X' \cup \{ x \} \in S_1$ by the augmentation property for the first matroid. In particular, $B \cup \{ x \} = (X' \cup \{ x \}) \cup Y' \in S$. So we have the augmentation property.
• Why should $A \cap X = X$ or $A \cap Y = Y$? Nov 5, 2017 at 0:33
• Oops, sorry. I miswrote. I should have said $A' = (A' \cap X) \cup (A' \cap Y)$. Now is it clear? Nov 5, 2017 at 1:07